U.S. patent number 3,766,470 [Application Number 05/146,256] was granted by the patent office on 1973-10-16 for apparatus for testing the integrity of a thru-hole plating in circuit board workpieces or the like by measuring the effective thickness thereof.
This patent grant is currently assigned to Unit Process Assemblies, Inc.. Invention is credited to William D. Hay, Robert Jensen.
United States Patent |
3,766,470 |
Hay , et al. |
October 16, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
APPARATUS FOR TESTING THE INTEGRITY OF A THRU-HOLE PLATING IN
CIRCUIT BOARD WORKPIECES OR THE LIKE BY MEASURING THE EFFECTIVE
THICKNESS THEREOF
Abstract
An apparatus is provided for testing the integrity of thru-hole
plating in printed circuit boards by measuring the effective
thickness thereof. A printed circuit board is adapted to be
positioned on a work surface of the apparatus with the plated
thru-hole to be measured disposed at an operating location. A
spaced pair of electrode probe are displaced downwardly along a
predetermined path into engagement with the top defining edges of
the plated thru-hole. A second spaced pair of electrode probe
elements engage the bottom defining edges of the thru-hole whereby
a predetermined magnitude of constant current can be passed through
one of the upper electrode probes, the plating and one of the lower
electrode probes and the voltage drop developed thereby across the
thru-hole plating measured by the other set of upper and lower
electrode probe elements. The upper and lower electrode probes are
suitably contoured to establish essentially point contact with the
surfaces of the thru-hole plating whereby accurate and reproducible
readings can be obtained.
Inventors: |
Hay; William D. (Peekskill,
NY), Jensen; Robert (Jamaica, NY) |
Assignee: |
Unit Process Assemblies, Inc.
(Woodside, NJ)
|
Family
ID: |
22516533 |
Appl.
No.: |
05/146,256 |
Filed: |
May 24, 1971 |
Current U.S.
Class: |
324/716;
324/715 |
Current CPC
Class: |
G01R
31/2805 (20130101) |
Current International
Class: |
G01R
31/28 (20060101); G01r 027/14 () |
Field of
Search: |
;324/64,71R,158P,158F |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
371,998 |
|
Mar 1923 |
|
DD |
|
569,174 |
|
Jan 1933 |
|
DD |
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250,321 |
|
Jun 1970 |
|
SU |
|
Other References
Curtis et al., 4-Point Probe Measurement, IBM Technical Disclosure
Bulletin, Nov. 1970, p. 1697..
|
Primary Examiner: Krawczewicz; Stanley T.
Claims
Having thus described our invention, we claim:
1. An apparatus for measuring the conductivity of a plated coating
on a thru-hole in a circuit board workpiece comprising
a planar supporting platform defining a measurement station and
adapted to support a circuit board workpiece having a thru-hole
with a plated coating disposed in overlying relation therewith,
a first pair of independently resiliently mounted displaceable
electrode elements disposed in closely spaced positional relation
beneath said platform and each adapted to engage a marginal
defining edge of the bottom of said thru-hole disposed at said
measurement station,
a second closely spaced pair of displaceable electrode elements
selectively disposed in longitudinal coalignment with said
electrode elements of said first pair thereof above said platform
and each adapted to engage a marginal defining edge of the top of
said thru-hole,
means for lineally displacing said second pair of electrode
elements intermediate a retracted position remote from said first
pair of electrode elements and an advanced position in operative
engaged relationship with the upwardly disposed defining edge of
said plated thru-hole on an interposed workpiece supported by said
platform, and
means for independently resiliently mounting said second pair of
electrode elements when in advanced position so that the plated
marginal defining edges of the thru-hole in the interposed
workpiece engaged by said first and second pairs of electrode
elements are subjected to a substantially uniform and reproduceable
magnitude of compressive force during the measurement
operation.
2. A measuring apparatus according to claim 1 wherein each
electrode element of said second pair of electrode elements
includes
cylindrical probe means having terminal contact segments
thereon,
each of said contact segments having contoured surface means for
selectively engaging the inner edge of the top surface of the
thru-hole plating being measured and defining, in a plane parallel
to a plane defined by the axes of said cylindrical probes, an acute
angle with the surface of an interposed workpiece and having, in a
plane perpendicular to said first mentioned plane, a circular
contour, and
wherein each electrode element of said first pair of electrode
elements includes
terminal contact segments having contoured surface means
defining, in a plane parallel to the plane defined by said axes, an
acute angle with the interposed workpiece and having, in a plane
perpendicular to said plane, a circular contour whereby when said
first and second pairs of electrode elements are brought into
engagement with the inner edges of the top and bottom surfaces of
the thru-hole plating, each of said first and second contoured
surface means will establish essentially point contact with the
thru-hole plating.
3. A measuring apparatus according to claim 3 wherein said terminal
contact segments of said first and second pair of electrode
elements define, in the plane parallel to the plane defined by the
axes of the cylindrical probes, symmetrical V-shaped and inverted
V-shaped terminal portions respectively and wherein the imaginary
bottom and top of the V and inverted V configurations respectively
are vertically aligned.
4. A measuring apparatus according to claim 2 wherein the lowermost
points of curvature transversely across the contoured surface means
of each of said first and second electrode elements define a
vertical plane.
5. A measuring apparatus according to claim 2 wherein the points of
contact between said second electrode elements and the top surface
of a thru-hole plating in a horizontally disposed workpiece are
vertically above the points of contact between said first electrode
elements and the bottom surface of said thru-hole plating.
6. A measuring apparatus according to claim 1 wherein said lineally
displacing means includes means for independently varying the
retracted position of each of said second electrode elements.
7. A measuring apparatus according to claim 6 further
comprising
means for biasing said first pair of electrode elements upwardly to
extend beyond the surface of said supporting means.
8. A measuring apparatus according to claim 7 further comprising
means for preventing the striking of a workpiece against said first
electrode elements when the workpiece is introduced into interposed
relation between said first and second electrode elements.
9. A measuring apparatus according to claim 8 wherein said
apparatus further comprises magnification means and illuminating
means disposed adjacent to said measuring station for facilitating
manual positioning of a plated thru-hole in a workpiece in the path
of advance of said electrode elements.
10. An apparatus for measuring the integrity and thickness of a
thru-hole plating according to claim 1, further comprising an
elongated housing, a first spaced pair of individually shielded
electrical leads joined to one set of said first and second
electrode elements and extending longitudinally of said housing in
parallel relation to each other and a second spaced pair of
shielded electrical leads joined to the other set of said first and
second electrode elements and extending longitudinally of said
housing in parallel relation to each other, said first and second
spaced pairs of shielded electrical leads being so positioned that
a plane containing said first pair of electrical leads will be
disposed substantially perpendicular to a plane containing said
second pair of electrical leads.
11. An apparatus for measuring the integrity and thickness of a
thru-hole plating according to claim 10 further comprising a
readout unit for supplying a constant electrical current to one set
of first and second electrode elements and means for measuring the
voltage drop across said other set of first and second electrode
elements, said means including a high gain input circuit feeding a
low noise amplifier arranged for common mode operation and
indicating means responsive to the output of said amplifier for
visually displaying the magnitude of said voltage drop.
Description
This invention relates to instruments for measuring the
conductivity or thickness of thin layers or coatings of
electrically conductive material, and particularly to instruments
for measuring the plating thickness or conductivity of electrically
conductive materials on the side walls of thru-holes in printed
circuit boards.
Beta ray backscatter measuring instruments have been extensively
utilized to measure the thicknesses of metallic deposits and
coatings of various materials. These instruments generally include
a source of beta radiation, conveniently a radioactive isotope.
This source emits radiation which is directed to strike a metallic
coating and the radiation backscattered from the coating is
measured by a detector in the form of a Geiger-Muller tube. An
associated electronic counter or readout unit converts the output
of the detector into utilizable intelligence. Such beta ray
backscatter measuring instruments cannot be used for measuring the
thickness of the plating in a thru-hole in a printed circuit board
where one base plating has an over plating of gold or solder.
Additionally, such beta-ray backscatter measuring instruments are
unable to detect cracks, voids, or other defects in the plated
coating.
The present invention may be briefly described as an apparatus for
non-destructive measurement of the physical integrity and thickness
of plated coatings on thru-holes in printed circuit boards by
micro-resistance measurement techniques. Upper and lower pairs of
elements are displaced into engagement with the top and bottom
surfaces of a plated thru-hole. A predetermined amount of current
is passed through one of the upper electrode elements , the plating
on the thru-hole and one of the lower electrode elements and the
voltage drop developed as a result of the passage of current across
the thru-hole plating is measured by the other upper and lower
electrode elements.
Among the advantages of the present invention is the provision of
an improved apparatus for measuring the thickness of thru-hole
plating in printed circuit boards as well as providing an
indication of the physical integrity thereof.
Accordingly, the principle object of the present invention is the
provision of an improved apparatus for non-destructively measuring
the thickness of thru-hole plating in printed circuit boards.
Another object of the present invention is the provision of an
apparatus which can reliably indicate the integrity and thickness
of copper plating in a thru-hole even in the presence of
overplatings of gold or tin-lead.
A further object of the present invention is the provision of an
apparatus which can detect cracks, voids, the presence of bath
contaminants and other defects in printed circuit board thru-hole
platings.
Other objects and advantages of the subject invention will become
apparent from the following portion of the specification and from
the accompanying drawings which illustrate in accord with the
mandate of the patent statutes presently preferred embodiments
incorporating the principles of the present invention.
Referring to the drawings:
FIG. 1 is an oblique view of the apparatus incorporating the
present invention.
FIG. 2 is a longitudinal section taken at the lines 2--2 of FIG.
3.
FIG. 3 is a plan view of the apparatus illustrated in FIG. 1 shown
with the top cover plate removed.
FIG. 4 is a skeletonized showing of certain of the apparatus
elements as illustrated in FIG. 2, with the upper electrode probe
elements disposed in operative relationship with a plated thru-hole
in a printed circuit board.
FIG. 5 is a vertical section taken on the lines 5--5 of FIG. 3.
FIG. 6 is an enlarged elevation of the electrode probe elements
disposed in operative engagement with a plated thru-hole in a
printed circuit board.
FIG. 7 is an elevation taken at the lines 7--7 of FIG. 5.
FIG. 8 is a block diagram of the major circuit components included
in the apparatus illustrated in FIG. 1.
FIG. 9 is a schematic view of the wiring system employed in the
probe stand.
FIG. 10 is a simplified wiring diagram of the electrical connection
to the electrode probe elements.
FIG. 11 is a view of the wiring arrangement as viewed from the line
12--12 of FIG. 11.
FIG. 12 is a simplified wiring diagram of a prior art technique for
feeding current to and measuring the voltage drop across a circuit
board thru-hole .
Referring to the drawings and initially to FIG. 1 there is provided
an apparatus 10 for measuring the effective thickness and for
sensing defects in the plated coatings of plated thru-holes in
printed circuit boards. As there shown, the apparatus 10 includes
an elongate rectangular base member 12 supported on corner
pedestals 14 which provides a horizontal workpiece supporting
surface 16. A pair of spaced lower electrode elements 18 are biased
to project upwardly through an aperture 19 in the base member 12
above the plane of the surface thereof and to define an operating
location. A printed circuit board (not here shown) is positioned on
the work surface and is supported by the work surface and by an
ease-on ramp 20 which is biased to project upwardly from the work
surface a distance which is sufficient to prevent the striking of
the lower electrode elements by the printed circuit board when it
is positioned on the work surface. The printed circuit board is
positioned with a plated thru-hole to be measured disposed above
the lower pair of electrode elements. An upper pair of electrode
elements 24 are vertically displaceable in an elongated housing 26.
When the control knobs 28 are rotated these electrode elements are
vertically lowered and are thereby brought into engagement with the
top surface of the thru-hole plating. Further lowering of the
spaced probes results in the ease-on ramp, which is normally biased
to project upwardly beyond the work surface, being forced
downwardly. As the ease-on ramp is displaced downwardly the lower
surface of the thru-hole plating engages the lower electrode pair
(unless the weight of the circuit board is sufficient to lower the
ramp bringing the circuit board into engagement with the lower
electrode element prior to the lowering of the upper electrode
elements) and further lowering of the upper electrode pair forces
the workpiece, ease-on ramp and lower electrode pair downwardly
until the workpiece is biased into planar engagement with the work
surface. The workpiece is accordingly pinched between the opposing
pairs of electrode elements thus assuring excellent contact between
the upper pair of electrode elements 22 and the top surface of the
thru-hole and electrode elements 18 and the lower surface of the
thru-hole plating being tested.
The elongated housing 26 is mounted to the back of the base 12 and
extends along the base in raised relation therewith to define a
work space therebetween open at the sides and front. The housing
includes laterally spaced side walls 30 which have depending
extensions 32 at the back thereof, a back wall 34 (FIG. 2) which
extends between the side walls 30 and removable cover plates 36, 38
which enclose the top and front end portions of the housing.
The testing apparatus is electrically connected to a readout unit
or meter 40 which supplies current to the apparatus which passes
through one set of upper and lower electrode elements. The voltage
drop across the thru-hole plating developed as a result of the
passage of current therethrough is measured by the second set of
upper and lower electrode elements and converted by the readout
unit into utilizable intelligence.
A guide body 42, (FIGS. 2 through 5) which may be made from metal,
synthetic resin or any other suitable material, extends between the
front end portions of the apparatus side walls 30. The guide body
has a pair of spaced parallel bores 44 which extend vertically from
the top surface to the bottom surface thereof. The upper pair of
electrode elements 24, which are cylindrical in shape, are inserted
into these bores and establish sliding engagement therewith.
A guiding element 46 is secured in a conventional manner to the
uppermost portion of each of the upper electrode elements. These
guilding elements, when so secured, have rectangularly shaped
projections 48 which are located within vertical channels 50
defined in the guide body and which communicate on either side
thereof with the guide body bores 44 and a guide body chamber 52.
These rectangular projections establish a sliding relationship with
the vertical channels 50 and accordingly prevent the rotational
displacement of the upper electrode elements once they are inserted
into the cylindrical guide body bores. A pair of stops 53 limit the
upward displacement of these electrode elements.
The vertical displacement of the upper electrode elements is
controlled by levers 54 which are rockable intermediate their ends
on a shaft 56 which horizontally extends between the housing side
walls 30. Forward end portions of the lever extend through the
guide body chamber 52. The rockable levers include slots 55 at the
front end thereof which engage the guide element projections 48 of
the upper electrode elements. An elongated block 60 is pivotally
supported on the shaft 56 and this elongated block extends
rearwardly from the shaft on either side of the rockable levers.
Tension springs 62 extend through bores 64 in the block and are
anchored at their opposite ends to a rod 66 which extends across
the top of the block and to projecting pins 68 integral with the
rockable levers whereby the right ends of the rockable levers are
forcefully and independently urged toward the block 60. Set screws
70 extend through the block and engage the rearward ends of the
rockable levers. As can be appreciated these set screws 70 enable
the upper electrode elements to be vertically adjusted so that they
will have the same vertical position relative to each other and
also enables the elements to be adjusted so as to assure maximum
elevation of the probes without the tops thereof striking the upper
stops 53. The block has a rearwardly extending arm 72 which carries
a rotatable cam follower 74 which engages a related cam 76 secured
to a cam shaft 78 journalled in the rear portions of the side walls
30. Tension springs 80 extend upwardly from the forward end
portions of the rockable levers to a member 82 which extends across
the top of the housing to urge the rockable levers and hence the
block to rotate in the clockwise direction as viewed in FIG. 2 and
thereby maintain the cam follower roller in contact with the
periphery of the cam. The lower surface of the rockable lever slots
engage the rectangular portions of the guilding elements 46 causing
the upper electrode elements to be elevated upwardly.
As the control knobs 28 are rotated the block and rockable levers
are rotated. The weight of the upper electrode elements maintains
the guide element projections 48 of these elements resting on the
bottom surface of the rockable lever slots 55 until these electrode
elements are lowered into contact with the circuit board W. Further
rotation of the control knobs brings the upper surface of the slots
into forceful engagement with the top surfaces of the guide element
projections whereby the circuit board is urged, as above described
into planar engagement with the supporting surface 16 of the
apparatus. The upper electrode elements are downwardly biased by
the stretched springs 62 and accordingly a predetermined,
controlled force can be achieved at the faces of the upper
electrode elements.
The spaced lower electrode elements 18 are secured to rod elements
86 which extend downwardly through and are secured to individual
insulating leaf springs 87. Cylindrical spring elements 88 are
compressively located intermediate the insulating leaf springs and
a shelf 94 which extends across the bottom of the base 16. The
resilient nature of the spring element 88 and the insulating leaf
spring 87 normally urges the lower electrode elements upwardly to
project beyond the top surface of the work surface. The shelf 94
acts as a lower stop limiting the downward displacement of the
lower electrode elements.
As an aid to positioning the workpiece W on the work surface the
ease-on ramp or guard 20 is provided which elevates the workpiece W
over the lower electrode elements 18. As can be seen from FIGS. 1
and 2 the ease-on ramp is pivotally mounted and is biased by a
spring element 102 which extends between one end of the ease-on
ramp and the base shelf 94 to rotate the end thereof adjacent the
lower contact pair upwardly to at least the height of the lower
electrode elements when the probe assembly is in the inoperative
condition. The force of the downwardly urged upper electrode
elements will be sufficient to lower the workpiece W, the upwardly
biased ease-on platform 20 and the upwardly biased lower electrode
elements 18 into the operational condition as illustrated in FIG. 4
with the workpiece in planar engagement with the work surface
16.
To achieve an extremely high degree of accuracy and reproducibility
in the desired measurements the upper 24 and lower 18 pairs of
electrode elements include contact members 22, 22' selectively
contoured to establish essentially point contact with the inner
diametral edges of the top and bottom surface of the thru-hole
plating 81 being measured. To accomplish this the contact surfaces
of the contact members are angled in the transverse direction (FIG.
6) relative to the plane of the work surface. This angle may be as
small as 5.degree. as illustrated or can be substantially larger
(for example 60.degree. or more) where required. An angle of
30.degree. is illustrated by the dotted line in FIG. 6. The
inclination of the upper pair of contact members establishes
therebetween a flattened V configuration and the lower pair of
contact members are similarly inclined establishing therebetween an
inverted flattened V configuration with the imaginary bottom and
top of the V and inverted V as well as the bottommost points of the
upper and lower contacts being vertically aligned. The longitudinal
cross-section of each of the contacts as illustrated in FIG. 7 is
circular and the lowermost points thereof extending transversely
across the upper and lower contacts lie in a single vertical
plane.
The transverse spacing between the two contacts of each pair may be
fixed and may be as small as desired. With thru-hole platings
having a diameter at least as large as the spacing between the
upper and lower contact pairs essentially point contact at each of
four spots on the sample - two on either face of the board at
opposite ends of a diameter of the thru-hole plating being measured
and precisely at the edge of the thru-hole will be established.
In order to obtain such highly accurate and reproducible readings,
it is important to position the probe assembly as accurately as
possible so that four point contact as above described will be
achieved. Accordingly, a light 110 and a magnifying lens 112
suitably secured to the testing apparatus are provided giving the
operator a large and illuminated view of the thru-hole being
measured. To this end the feet of the upper electrode elements are
inclined downwardly from front to back so that they will not
obstruct the view of the upper contacts.
Referring to FIG. 8 the input circuit is a high gain, low noise
operational amplifier 117. The circuit is arranged for common mode
operation with a medium input impedance and a typical gain of 175.
The circuit board being tested is accordingly immune to voltage
potentials between the circuit board and ground and also to any
other electrical noise in the air. The leads 114 running from the
testing apparatus 10 to the readout meter 40 are twisted pairs with
the voltage measuring leads being twisted separately from the
current inducing leads to eliminate any pickup between the input
and output leads.
An internal shunt 118 is positioned in series with the input
current inducing lead which is utilized to calibrate the system. A
spring loaded switch 120 enables the operator to switch the
amplifier input from the electrical contacts to the know or
standard resistance internal shunt output. The read-out unit 40 is
then set by means of the calibration control to directly read the
resistance of the internal shunt. A thru-hole plating of the
circuit board can then be measured.
The meter circuit includes a high gain operational amplifier (not
shown) 122, which is of conventional construction, desirably in
which the meter is connected in the feedback loop through a Diode
bridge. The initial gain of this stage typically is 1000 until the
output reaches the breakover point of the diodes in the bridge. At
that time the gain is reduced typically to two and one half. This
enables linear readings from zero to 100 micro-amperes without
having to offset the meter zero reading.
The current and voltage leads are twisted from the plug of the
cable up to the probe stand or housing and are then run in steel
shielding tubes 116 (FIG. 9) and wired diagonally opposite each
other (FIGS. 10 and 11), i.e. the voltage leads define a plane
which is substantially perpendicular to the plane defined by the
current leads. This technique as compared to prior art practice
illustrated in FIG. 12 substantially eliminates all errors in
measurement arising from transformer action (the coupling through
the air of the current loop to the voltage pick-up loop.)
* * * * *